Molecular jet separators are typically used for separating a carrier gas from a sample in gaseous form, for further analysis. In an important application, a molecular separator is connected to interface the output of a gas chromatograph to the input of the ion source of a mass spectrometer. The separator is said to enrich the sample and it is desirable that the separator achieve maximum enrichment as well as optimum yield of the sample. By "yield" is meant the proportion of the sample transferred from the inlet to the outlet of the separator.
A known form of molecular jet separator is disclosed in U.S. Pat. No. 3,957,470 to Dawes. This separator comprises an integral glass component which includes a pair of accurately aligned input and output tubes with a fine evacuation gap between their opposed open ends, and a surrounding envelope which defines an evacuation chamber and includes a tubular outlet for connection to a vacuum pump. This design had the advantage that it could be formed from a single piece of glass tubing which is either bridged to ensure alignment when a gap is cut fully through the tubing, or is provided with a transverse slot to form the gap. A jet orifice may be provided at the gap in the inlet segment of the tubing.
A separator of the general type disclosed in U.S. Pat. No. 3,957,470 has proven satisfactory for some years but has been found to have a number of limitations which prevent its optimum application with modern capillary column gas chromatographs. These existing molecular jet separators acquire a minimum flow typically in the range 20 to 50 mls per minute, a limitation which arises in part from the minimum gap width of about 100 micron and from a minimum inlet orifice at the gap of a similar order. Capillary column flows are typically substantially less than 20 mls per minute and one practice adopted to overcome the problem has been to augment the flow from the gas chromatograph with additional carrier gas upstream of the separator. This practice is somewhat counterproductive and tends to reduce the resultant yield of the separator.
Prior one-piece separators have two other disadvantages which arise from their physical form. Where it is desired to provide for varying jet orifices and/or evacuation gaps to suit different input gasses or input sources, the whole unit must be changed over. There is also a problem in cleaning the units if they become blocked in the region of the jet orifice or gap.